Carbon dioxide freezeout system



D. c. JENNINGS 3,103,427

CARBON DIoxIDE FREEZEOUT SYSTEM 5 Sheets-Sheet 1 Sept. 11o, 1963 FiledJime 28, 1960 INVENTOR y AGENT 1////c avra/e w,4

DAVID C. JENN/NGS Sept- 10, 1963 D. c. JENNINGS 3,103,427 CARBON DIoxIDEFREEZEOUT SYSTEM /NvE/vron DA wo c. JE/v/v//vcs l AGENT Sept 10, 1963 b.lc. JENNlNGs 3,103,427

CARBON DIOXIDE FREEZEOUT SYSTEM INVENTOR DAI/ID C. JENN/NGS AGENT UnitedStates. Patent O This invention relates to an environmental controlsystem for a closed compartment which is occupied by one lor morehumanbeings, and more panticularly to a system for extracting carbondioxide from the? artiiicial atmosphere of such a sealed cabinl in orderto lim-itthe concentration of carbon dioxide to a level compatible withsupport of human life. It is fundamentally important in such a sealedcompartment, as, for example, in an outer space vehicle, to providemeans forreconditioning the air so as to preserveA a habitableatmosphere with regard v to physical state and chemical composition, andan essential requirement is found in the need to remove carbon dioxideat a rate substantially equivalent to the rate at which it is generatedby the human occupants.v Y

Industrial operations, particularly those concerned with the synthesisor purication of certain gas mixtures of gases, have employed Varioustypes of systems for removal of carbon dioxide, including methods ofchemical adsorption, adsorption, liquefaction, and freezeout, but theirconcern has been primarily with removal of carbon dioxide withineconomic considerations pertinent to operations on or near the surfaceofthe earth,`where power required and equipment weight are not of thesame degree of importance as in a space vehicle. Therefore, it is a-nobject of this invention to provide-a carbon dioxide freezeout systemwhich lendsitself particularly to the requirements of a space vehicleand is characterized by high thermodynamic efficiency, low powerconsumption, and inherently low system weight so as to avoid unduepenalties to the overall performance of ay manned space vehicle.

It is still a further object of this invention to remove carbon dioxidefrom the contained atmosphere by a system as described at a ratesubstantially equivalent to the rate at which it is being introducedinto the closed comy partment atmosphere by the exhalations of the humanoccupants, that is to say that the concentration of carbon dioxide willnot for any significant period ofY time exceed a previouslyestablishedmaximum allowable concentration level. This may beaccomplished either by continuous or intermittent operation tof thesystem.

It is an important feature of this invention to utilize the heat whichis extracted from the processed `atmosphere in reducing its temperatureto the range in which carbon dioxide will be frozen for reheating thisprocessed atmosphere after carbon dioxide separation and prior toreturning it to the closed compartment.

It is a still further object to provide a system in which the processedatmosphere is used as 4the refrigerant uid for abstraltcing heat fromthe gas cooled and delivering it tio the returning gas or other meansfor heat rejection at temperatures higher than the freezing temperaturerange of the carbon dioxide removed. l

It still is a further object of this invention to provide a system thatis unaifected by the gravitational problems normally associated with azero gravity environment.

It still is a further object lof this invention to provide arefrigeration system for removing carbon dioxide from an 4atmospherewherein a pair of carbon dioxide freezers 3,103,427 Patented Sept. 10,1963 ice cycle and having a connection for alternately supplyingcompressor discharge air to the carbon dioxide freezers for acceleratingsublimation and a second connection for utilizing a portion of the heatgenerated by the compressor for heating the carbon dioxide free air.

Upon reviewing the description which is made in conjunction with thefollowing drawing, one skilled in the art will readily realize thatvarious modifications may be made without departing from the spirit andscope of this invention. p n' n FIG. l diagrammatically illustrates apreferred embodiment of this invention including a system for removingboth water and carbon dioxide. l

- y freezer systems to com-plete its cycle.

A to 4this invention as will be more fully appreciated from ,the

are alternately connected to a pair of heat regenerators FIG. 2 is apartial showing iofya schematic illustration of another embodiment ofthis invention.

FIG. 3 diagrammatically illustrates another preferred embodiment whichutilizes a pair of regenerat-or-s. automatically connected with a pairof carbon dioxide freezers.

FIG. 4 diagrammatically illustrates this invention and utilizes aplurality [of space radiators which form or are contaminated by theexhalation of the occupants within the cabin is extracted by compressor16 which is driven by the combination `of motor 18 and expander 20 whichmay take the form of any of the well known positiveor non positivedisplacement expansion motors. As will be appreciated from the followingdescription, the compressor and expander are connected in bootstraprelationship, that is to say that the air which is being compressed is`subsequently used to propel the expander in such a manner thatv energyis expended from the compressed air so asv to be adiabatically expanded,which results in a substantial temperature drop. Of course, la portionof the power therefore is supplied by the expander while the remainderportionmay be supplied by an electrical motor or any other suitablemeans. As the air leaves the compressor, it is precooled Iby primaryheat exchanger 24 and directed therefrom to the water freezer by 4-Wayvalve 3,10 which connects line 26 to line 28. It will be realized thatywhile one set of freezers is utilized for removing water.v and carbondioxide from the air stream, the second set is being regenerated andheld in a stand-by condition until a predetermined time sopas to alloweither one of the The method for automatically cyclically connecting thewater freezers to the carbon dioxide freezers is well known in the art,while it should be realized that connecting the carbon .dioxidedischarged air'to water freezers is an important feature description tofollow.

Still referring to FIG. 1, the precooled compressed air is passedthrough passage 36 of heat exchanger 32 for melting the ice which hasaccumulated from the previous cycle and leaves .therefrom 'at atemperature, say 35 F. The moisture is in turn fed to moisture separator38 where the moisture is collected in anyy of the well-known types andin such a mannerv so as not to interfere with the air stream which isdirected to passage 40 of heat exchanger 34 for further cooling of theair. The purpose of this operation is to assure that the air fed to thecarbon dioxide freezeout system is substantially free of all of `itsmoisture content. t' Owing to the fact that` the air in passage 50 whichhas previously been in communication with the carbon dioxide freezers isat a J3 a low temperature; the temperature of .the air in passage 40which is in heat transfer relation thereto will be reduced to say, 150F. The air, now essentially free of its moisture, is then fed to passage60 of. heat exchanger 58 which is in out-of-contact heat relation withthe uid in passage 62 which has accreted with frozen carbon dioxide fromthe previous cycle. The temperature of the air in passage 60 isdecreasing, duejto the transfer of heat to passage 62 which causes therate of sublimation of the carbon dioxide to increase. The sublimedcarbon dioxide is then vented out of the system to a low pressure orzero pressure environment via line 64 and 63. The now colder air isadmitted to freezer 42 where a major fraction of the .carbon dioxide inthe 'processed air freezes and accretes within passage 66. The air whichis now at a low temperature say, 240 F., is fed to passage 50 of freezer34, taking up some of the heat released by freezing the water in passage40 as previously mentioned and then fed through regenerator heatexchanger 61S for recooling the air and then to expansion motor '20 forfurther cooling of the air. The air is then delivered to freezer 42vwhere it is utilized for` bringing down the temperature of the aircontaminated with carbon dioxide in passage 66 for the purpose ofcondensing and separating carbon dioxide as previously mentioned. Theair is then delivered to the cabin via passage 70 passing throughregenerator 68 and primary heat exchanger 24 and picking up heat throughp each one of these devices.

From the foregoing, it is apparent that a large portion of the energywhich has been expanded in lowering the temperature of the carbondioxide and water contaminated air is regained by precooling .the airstream at various locations in the cycle, prior to readmittance to thecabin. Although one shown, the water collected in the moisture'separator38 may be ejected into thejcabin for rehumidiiication purposes orcollected and used in liquid form. It is an important feature of thisinvention that the sublimated carbon dioxide is venting withoutincurring a loss of water, which in a space vehicle, is a valuablecommodity. l

`Upon reaching a predetermined load in the freezers, the activatedfreezers may be automatically or cyclically connected in the followingmanner. Valves 30, 31, 33', 35, 37, 39 'and 49 are caused to rotate forreversing the flow path. Although it is to be understood that the waterfreezeout system and carbon dioxide freezeout system may be cycledindependently of each other, the air discharging from primary heatexchanger 24 is directed to passage 40 of freezer 34 and melting frozenwater which had accumulated therein and passing the moisture laden waterthrough the moisture separator 38 and then to passage 36 of heatexchanger 32 and delivering the air at a lower temperature throughpassage 72 of freezer 42. At this point the air rejects heat from thecarbon dioxide laden passage 66 which now provides means for increasingthe rate of sublimation therein so as to increase the process of ventingthe carbon dioxide via passages 7-6 and 78 to a low or zero pressureenvironment. The now cooler air discharging from freezer 42 is deliveredto passage 62 of freezer 5S where heat is absorbed therefrom causingcondensation of the carbon dioxide within the chamber of the freezer.The carbon dioxide free air is then delivered via passage S0 to passage46 of heat exchanger 32 which supplies the low temperature sinkfor/bringing the dew point temperature of the moisture laden air inpassage 36 to say, --l50 F. At this point the air discharging frompassage 46 is slightly warmer and is again precooled at regenerator 68and delivered to expander 20 for further lowering of its .temperatureprior to delivery to passage 60 of heat exchanger 53. From this pointthe air is then redelivered to the cabin via line 70, regenerator 68,heat exchanger 24, and line 14 absorbing heat in each one of these heattransfer devices. Reversing exchangers and .their operation are Wellknown in the art and may be operated either manually or automatically.

Another exemplified system is shown in FIG. 2 which is substantiallysimilar to FIG. 1 which has been modied in its preferred form in such amanner as to more effectively utilize the heat transfer effectiveness ofthe water freezers. As will be noted as shown in FIG. l on alternatecycles, passages 46 and 50 of heat exchangers 34 and 36 respectivelyhave been dead-ended. In order to utilize the heat transfereffectiveness of these heat exchangers, the ilow path of the embodimentshown in FIG. 2 has been arranged so that the fluid is continuouslyilowing through these passages in such a manner as to be in heattransfer arrangement so as to precool the air prior to freezingthemoisture content. As will be noted, another valve and associatedconnecting conduits have been included to divide the flow in line 14 forbleeding cool air via lines 53 and 71 and alternately through lines 53and 73 so as to take advantage of the low heat content within the waterfreezers. It will also be noted that the manner of feeding the airstream' to the water freezers has been modified by alternatelyconnecting the heat exchanger discharge air in line 26 to passages 46and 50 in freezers 32k and 34 respectively. It will further be notedthat these passages are alternately connected to the moisture separator.Thus, during one cycle of operation a portion of the puried airdischarging from regenerator 68 is diverted through line 57, valve 41,through passage 36 of water freezer 46 and returned toline 14 just priorto being injected into the cabin by way of valve 30. In alternating thisflow path so as to take advantage of the cold temperature air forprecooling the cabin discharge air valves `41, 45, 43, and 30 are causedto rotate. Thus, the portion of purified air discharging fromregenerator 68 is conducted through line 57, valve 41, "passage 4t) ofwater freezer 34 and returned to line 14 via valve 30.' This arrangementmakes possible the use of a much smaller and consequently lighter weightheat exchanger generally indicated by numeral 13. This heat exchangermay be entirely eliminatedbut is shown in this particular embodimentbecause with its inclusion the component units such as the freezers andexpanders may be utilized at lower efficiencies and also making thesystem operable over a wide range of temperature conditions.

In accordance with this invention, an example of the condition of theair as evidenced in each component is included hereinbelow forillustration purposes. The following will be evidenced when thecompressor efficiency is the expander efficiency is 85%, regenerativeheat exchanger effectiveness is 90%, and the freezer effectiveness is90%. Referring to the embodiment shown in FIG. 2 in conjunction with theembodiment of FIG. 1 and assuming the following conditions to yexist inthe cabin.

T is the temperature in degrees Fahrenheit.

P is the absolute pressure in pounds per square inch.

W1 is the weight percentage of carbon dioxide in contaminated air.

W2 is weight percentage of water in the contaminated alr.

The condition of system components are: At the compressor discharge- Atprimary heat exchanger discharge- At the inlet of water freezer passage50- T:35.0 P: 13 .78 W2:0.455

At the discharge of the same- T: 160.0 P 13 .5 8 W2=Essentially 0 At thedischarge of passage 60 of freezer 58- T: 179 .O P: 13 .68

At the discharge of passage 66 of freezer 42 T: -206.0 P: 13 .48 W1:0.47

At the discharge of passage 40 of freezer 34- At the discharge ofpassage 69 of regenerator 68- At discharge of expansion motor 20'- Atthe discharge of passage 72 of freezer 42 I T: 183 .9

At the discharge of passage 71 of regenerator 68- At the discharge ofpassage 36 of freezer 32- 1 T:209.5

From the foregoing example, it will be apparent that the temperaturedifferential across the heat transfer portion of all the freezers, heatexchangers, and regenerators, which are all in their preferred form ofthe counter flow type, is at a substantially low value so that theefficiency of the respective heat transfer devices will evidence aminimum of entropy rises. As a result, a substantial reduction of thepower requirements is realized.

Referring to FIG. 3 which shows another exemplary embodiment of thisinvention which particularly includes a pair of regenerators 110 and 112which are alternately connected to freezers 114 and 116 and cyclicallyfreezing carbon dioxide and venting the sublimed carbon dioxide. Tofacilitate in the explanation of this embodiment, explanation of themajor parts which have already been described in the above, have beenomitted. Lt will be noted that a space radiator, which may be mounted sothat one of its surfaces forms a part of the outer vskin of a spacevehicle or projecting outwards from the vehicle, is utilized forextracting a portion of the heat in order to reduce the temperature ofthe air prior to delivery to the regenerator. The regenerator serves toextract heat from the air passing therethrough and retaining this heatin a manner well known in the artand then delivering the lowertemperature air to carbon di-k oxide freezer. In one cycle, for example,carbon dioxide in the air in the carbon dioxide freezer 116 is condensedand the solid carbon dioxide accretes on the wall of the freezerwhereupon carbon dioxide free air is then delivered to the expander,where its temperature is reduced and passed through passage 118- whichis in outof-contact heat relation to passage 120 for absorbing theenergy for condensing the carbon dioxide. The air is then alternatelypassed to the regenerator which has previously been heated and thenreturned to the cabin via passage 121. Simultaneously, the inactivefreezer 114 is connected to the compressor discharge air via passage 122where it mixes with the carbon dioxide free air in line 121 by way ofline 124 which was in communication With passage 126 of freezer 114. Atthis point the warmer air serves to increase the rate of sublimation ofcarbon dioxide which has accreted in freezer 114. When the temperatureand pressure of the freezer has reached a predetermined value, theoperation is switched so that the carbon dioxide freezer which hascollected frozen carbon dioxide is now sublimated and vented out to alow or zero pressure environment, and the freezer` which has previouslybeen ventedv isA now connected so as to collect frozen carbon dioxide.

Thus viewing the system as is shown in FIG. 3, cabin air is induced fromthe cabin through the suction line 130 and delivered to the compressorwhere it is slightly pressurized. However, as was'explained in theabove, the compressor also serves to load the expander. Compressordischarge air is then delivered to the space radiator through line 132and then to regenerator 110 via line 134. The regenerator is connectedto carbon dioxide freezer 116 by lines 136 and 138 where the carbondioxide, due to the extreme low temperature of the carbon dioxidefreezer, is extracted. The substantially carbon dioxide free air is thendelivered to the expandervthrough lines 140 and -142 where it is furthercooled andvthen delivered to passage 118 of freezer 116 Vthrough lines114 and 146. It will be appreciated that the air in passage 118 which isin indirect heat exchange relation with passage tends to cool the air inpassage 120 while tending to warm the air in passage 118. The dischargeair leaving passage 118 is then directed to the other regenerator forregaining heat through lines 148, 1,50, and 152 and from there theresidual air which is now free from carbon dioxide is returned to thecabin through lines 154 and 121.

It will be appreciated that during this cycle of opera# tion passage 120of carbon dioxide freezer 116 `was collecting carbon dioxide. Passage156 of carbon dioxide freezer 114 which had accreted with carbon dioxidein a similar manner in the previous cycle is during this cycle venting.This is accomplished by connecting passage 156 f of freezer 114 to alower pressure which in a space application would be the atmosphere byway of lines 158 and 160. In accordance with the invention, to increasethe rate of sublimation and hence the Idischarging of carbon dioxide,compressor discharge air which is the warmest air inthe cycle, is passedthrough lines 122, 162, 164, and branch line 124 which discharges thecompressor discharge air into the air returning to the cabin. The amountof air bled off from the compressor is proportioned to accomplish thedesired rate of sublimation.

To accomplish regeneration of freezers and regenerators, the flow pathis alternated by rotation of valves 166, 170, 172, 174 and 176 so thatin this manner the flow path is as follows: Cabin discharge air in linepasses through the compressor and the space radiator .through line 132and valve 176 which now connects the space radiator to regenerator 112which in turn is connected to passage 156 of freezer 114 through lines152, 180,' and 1160. From there the carbon dioxide free air is deliveredto the expander through lines 158 and 142 and returned to freezer 114through line 162 where itis placed in indirect` heat exchange relationwith passage 156 by passing through passage 126. From freezer 114 theair is delivered to regenerator 110 throughv line 164, valve 168, line154, and valve 166, and line 136, and then returned to the cabinAthrough line 134, valve 176, and line 121. Thus while this cycle isaccreting carbon dioxide in freezer 114, the carbon dioxide accreted infreezer 1,16 isnow vented. overboard through passage 7 and valve 172 andpassage 138 and valve 170. Again to increase the rate of sublimation,the compressor discharge air is directed through line 122, valve 176,and line 146 to passage 118 or freezer 116. From there the now coolerair is returned to the cabin through line 148, Valve 168i, branch line124, and line 121.

FIG. 4 is another system for removing carbon dioxide by a freezingprocess wherein the sublimed carbon dioxide is vented out of the systemat a rate substantially equal to the rate at which it is beingintroduced to the cabin. The radiators, which may be mounted to theouter skin of the outer space vehicle, are ideally suited for thisparticular application as is illustrated by the principle stated in theStefan-Boltzmann law wherein heat is radiated at the rate in accordancewith the following expression:

Q :EA T4 where E is an emissivity coefficient A is surface effectivearea T is absolute temperature O is a fundamental numerical constantHence, if a gas is passed through a radiator which can reject heat tospace and is insulated from other sources of heat, the gas will becooled. When the liquefaction (or sublimation) temperature of a chemicalconstituent of the gas is reached, that constituent will change itsstate upon further cooling and the frozen constituents will no longer beassociated with the residual gas. This type of gas purification processis particularly appropriate to a space vehicle, since, except in thedirection of the sun or a planet, the heat receiver is essentially allspace to infinity, with a receiver equivalent temperature of zeroabsolute.

Thus, carbon dioxide and water may be condensed from vapor to solidstates, whence they will be separated from the cabin atmosphere bypassage through the radiator.

The precooler(s) and the compressor-heat exchangerexpander unit permit areduction in the amount of heat which will need to be abstracted byradiation, hence realizing a reduction in the size of the radiator, aswell as providing the possibility of separately recovering and disposingof the water and the carbon dioxide. The radiators may be connected inparallel relation to each other so as to be adapted for automaticallyactuating the one exposed to the lowest temperature.

Reference is hereby made to U.S. application Ser. No. 339,385, filed byWalter E. Arnoldi, on June 28, 1960, and U.S. application Ser. No.39,366, filed by Gorken Melikian and George Peters, on `lune 28, 1960,and assigned to the same assignee, which applications are directed tothe systems shown in FIGS. 1, 2 and 4.

What has been shown in this invention are systems which are capable ofremoving water and carbon dioxide at a rate substantially equal to therate in which it is being ejected into the system by the exhalations ofthe human occupants occupying the cabin. Since the heat which has beenextracted for condensing the carbon dioxide is again utilized forreheating the air in its return to the cabin, a lightweight system isevidenced and the power consumed by the operation whether to be forcontinuous or intermittent operation is held at a minimum, therebyassuring a minimum penalty from both a weight and power standpoint.

It will further he realized that most of the moisture entrained in theair is removed prior to delivery to the carbon dioxide freezers and insuch a manner that the water will not be ventedalong with the carbondioxide'. Also, this permits the use 'of a smaller 'and consequentlylighter weight heat transfer device. It should be understood that theinvention is not limited to the particular embodiments shown anddescribed herein, but that various changes and modifications may be madewithout departing lfrom the spirit or scope of novel concept as definedhy the foiiowing claims.

I claim:

1. In a system for removing carbon dioxide mixed in air contained in asealed cabin, including a compressor, a nrotor along with an expanderfor driving said compressor, means for freezing carbon dioxide includingfirst and second heat exchangers, a pair of regenerators, a radiatormounted on =the wall of said cabin, means defining la flow path fromsaid cabin, to said compressor, to said space radiator, to one of saidregenerators, to said first heat exchanger, to said expander, back tosaid first heat exchanger, to the other of said regenerators and yblack.to said cabin, means for snblimating and venting the frozen carbondioxide in said first heat exchanger, means for accelerating thesnbliination of the carbon dioxide in said first heat exchanger, saidlast mentioned means ycomprising passage means and valving meansdisposed therein connecting the downstream side of the compressor ytosaid first heat exchanger.

2. In a system for removing carbon dioxide mixed in air contained in asealed cabin, including a compressor, a motor along with an expander fordriving said compressor, means for freezing `carbon dioxide includingfirst and second heat exchangers having rst and second passageways, fapair of regenerators, a radiator mounted on the wall of said lcabin,means defining a fiow path from said cabin, to said compressor, to saidspace radiator, to one lof said regenerators to said first passageway ofsaid first heat exchanger, to said expander, to said second passagewayof said first heat exchanger, to the other fof Said regenerators and tosaid cabin, means for alternately connecting said first passageway ofsaid second heat exchanger to said expander, means defining la passagedior alternately connecting fthe downstream side lof said compressor tosaid first or said second heat exchangers and means for simultaneouslyventing the first passageway of the first or second heat exchanger whenit is disconnected from said expander.

3. A system for removing carblon dioxide from 'air coniained in lasealed cabin subject to the exhalation of its occupants, said systemcomprising, in combination, inlet and outlet passage means connected tosaid sealed cabin, means including first and second carbon dioxidefreezers each having rst and second passageways =con nected to andlocated between said inlet and outlet passage means for separating thecarbon dioxide from the air, first and second heat transfer devicesyalso connected to and located between said inlet and outlet passagemeans and connected to said carbon dioxide freezers, means foralternately interconnecting said first and second heat transfer devicesto said -first 'and second carbon dioxide freezers, said first andsecond heart transfer devices each containing energy storing means, anexpand- `er alternately connected to thek second passageway of saidcarbon dioxide freezers for reducing the temperature olf said carbondioxide freezers to a value for condensing the carbon dioxide,conducting means for passing the air discharged from said secondpassageway of said carbon dioxide freezers to said energy storing meansto lower its temperature, a compressor rbeing driven by said expanderlocated in said outlet passage for extracting Aair from the cabin, saidcompressor serving to direct during an alternate cycle the air from saidcabin, to said compressor, to said first heat transfer devices, to therst passageway of said first carbon dioxide freezer, to said expander,to said second passageway of said first carbon dioxide freezer to saidsecond heat transfer device and back to said inlet passage means, fairbleed means interconnecting the downstream side of said compressoralternatedy to said first and second carbon dioxide freezers fordirecting a portion of compressor discharged air thereto, and means forventing the carbon dioxide during said alternate cycle from the firstpassageway of said second carbon dioxide freezer toa substantially lowpressure source.

4. An environmental control system for an outer space vehicle containinga sealed cabin having air subjected to receiving ia varying amount ofcarbon dioxide, said system `comprising a radiator, .a rst canbondioxide freezer having rst and second passageways, a first regenerato-rconnected between said radiator and said rst carbon dioxide freezer, asecond rregenerator connected between the :cabin and said rst canbronydioxide freezer, a second oanbon dioxide freezer ihaving rst and secondpassage- Ways mounted in parallel relationship to said rst carbondioxide freezer land adapted to be yalternately connected to either therSt regenerato-r or the second regenerator, an expander for cooling theair [located between said rst and second regenerators and said first andsecond canbon dioxide freezers and alternately connected to said rst`and second carbon dioxide freezers, means for deining a iicw path forsuccessively passing air from said cabin through said space nadiator,through said irst regenerator, alternately through the first passagewayof said irst and second carbon dioxide freezers and through said eX-pander, alternately through said second passageway of said rst andsecond carbon dioxide freezers, through said second regenerator and backto said cabin, uid passage means alternately connecting the downstreamside of the compressor, to said second passageway olf said 10 first andsecond carbon dioxide freezers, then to said cabin, and means foralternately venting to a substantially low pressure said iirstpassageway of said rst yand second carbon dioxide freezers wlhen eitherof said freezers is connected to said downstream side of the compressor.

References Cited in the file of this patent UNITED STATES PATENTS1,914,337 Belt June 13, 1933 1,949,616 Messer Mar. 6, 1934 2,022,782Pollitzer Dec. 3, 1935 2,039,889 De Baufre May 5, 1936 2,089,558 KlarwatAng. 10, 1937 2,097,434 De Bauiire Nov. 2, 1937 2,116,191 De Baufre May3, 1938 2,256,421 Borchardt Sept. 16, 1941 2,641,114 Holthaus June 9,1953 2,861,432 Haselden Nov. 25, 1958 FOREIGN PATENTS 511,567 GermanyOct. 31, 1930 625,815 Canada Aug. 15, 1961 644,139 Germany Apr. 24, 1937908,021 France Apr. 6, 1945 1,201,884 France July 15, 1959

1. IN A SYSTEM FOR REMOVING CARBON DIOXIDE MIXED IN AIR CONTAINED IN A SEALED CABIN, INCLUDING A COMPRESSOR, A MOTOR ALONG WITH AN EXPANDER FOR DRIVING SAID COMPRESSOR, MEANS FOR FREEZING CARBON DIXIDE INCLUDING FIRST AND SECOND HEAT EXCHANGERS, A PAIR OF REGENERATORS, A RADIATOR MOUNTED ON THE WALL OF SAID CABIN, MEANS DEFINING A FLOW PATH FROM SAID CABIN, TO SAID COMPRESSOR, TO SAID SPACE RADIATOR, TO ONE OF SAID REGENERATORS, TO SAID FIRST HEAT EXCHANGER, TO SAID EXPANDER, BACK TO SAID FIRST HEAT EXCHANGER, TO THE OTHER OF SAID REGENERATORS AND BACK TO SAID CABIN, MEANS FOR SUBLIMATING AND VENTING THE FROZEN CARBON DIOXIDE IN SAID FIRST HEAT EXCHANGER, MEANS FOR ACCELERATING THE SUBLIMATION OF THE CARBON DIOXIDE IN SAID FIRST HEAT EXCHANGER, SAID LAST MENTIONED MEANS COMPRISING PASSAGE MEANS AND VALVING MEANS DISPOSED THEREIN CONNECTING THE DOWNSTREAM SIDE OF THE COMPRESSOR TO SAID FIRST HEAT EXCHANGER. 